chore: Minor rbac memory optimization (#7391)

* test: Add benchmark for static rbac roles
* static roles should only be allocated once
* A unit test that modifies the ast value should not mess with the globals
* Cache subject AST values to avoid reallocating slices

coderd/rbac/authz.go

@@ -49,6 +49,20 @@ type Subject struct {
 	Roles  ExpandableRoles
 	Groups []string
 	Scope  ExpandableScope
+
+	// cachedASTValue is the cached ast value for this subject.
+	cachedASTValue ast.Value
+}
+
+// WithCachedASTValue can be called if the subject is static. This will compute
+// the ast value once and cache it for future calls.
+func (s Subject) WithCachedASTValue() Subject {
+	tmp := s
+	v, err := tmp.regoValue()
+	if err == nil {
+		tmp.cachedASTValue = v
+	}
+	return tmp
 }
 
 func (s Subject) Equal(b Subject) bool {